1. Field of the Invention
The present invention relates to a laser microscope and a confocal laser scanning microscope.
2. Related Background Art
When a sample with a fine structure is to be observed by a microscope, the critical resolution of the microscope can be expressed by the following: EQU .delta.=.lambda./2NA (1).
In this expression, .delta.represents the resolution, .lambda. represents the wavelength in use, and NA represents the numerical aperture of an objective lens, respectively.
From the above expression (1), it is suffice in order to enhance the resolution, if the wavelength .lambda. is reduced, or the numerical aperture NA of the objective lens is enlarged, or the wavelength .lambda. is reduced while the numerical aperture NA of the objective lens is enlarged.
When a living sample is to be observed, if the living sample receives a light with a wavelength in the ultraviolet range or lower, the sample itself is damaged due to a photochemical reaction, or the like, so that the resolution can be enhanced by using an objective lens of a liquid immersion type having a large numerical aperture NA.
On the other hand, an inorganic sample (for example, a fine structural member such as an IC) is to be observed by means of an objective lens of the liquid immersion type, impurities may attach to the surface of the IC, or the like. In this case, it is highly probable that the microscope can not be used properly. Thus, the resolution is enhanced by irradiation of a light in the ultraviolet range or lower. In this respect, the inorganic sample itself is seldom damaged by the irradiation of a light in the ultraviolet range or lower.
Incidentally, since the scale of a fine structural member such as an IC is recently rapidly diminishing, it is also demanded to further enhance the resolution of a microscope. Though there exists conventionally a microscope employing an X ray or electron beam as a microscope of high resolution, such microscope is to observe the surface of a sample in a vacuum and the operability thereof is not satisfactory, compared with that of an optical microscope. It is therefore required to enhance the resolution by the use of an optical microscope with an excellent operability.
In order to enhance the resolution in an optical microscope, for example, by two times, if selecting the wavelength of the deep ultraviolet range (273 nm), which is a half of that of an e line (the wavelength of 546 nm) as the representative wavelength of a visible light, a design of the objective lens becomes difficult for the following reasons.
That is, the objective lens is often constituted by combining single lenses with each other since the achromatic method by bonding lenses of different glass materials together which is normally employed in designing a conventional objective lens can not be used, because there is no such adhesive as is not changed in quality in the deep ultraviolet region or there is few glass materials having a sufficient transmittance in the deep ultraviolet region.
In this case, when half width half maximum of the wavelength distribution with respect to the central wavelength is not in the order of picometer (pm) or less, a predetermined optical performance can not be obtained. As a result, an image with an excellent resolution can not be obtained.
It is known that a light source employing a mercury lamp or a xenon lamp also emits a light with the wavelength shorter than the ultraviolet region. However, when only a light having a wavelength with half width half maximum in the order of pm or less is taken out by an interference filter to be used as an illumination light, there arises a problem that a required brightness can not be obtained by means of a conventional image pickup tube or CCD which has no sufficient sensitivity in the deep ultraviolet region.
To cope with this problem, a storage time can be prolonged to securely obtain the required brightness or the half width half maximum can be expanded in proportion to the sensitivity. However, in this case, an image acquiring rate may be sacrificed or the optical performance may be deteriorated.
Accordingly, in order to observe an inorganic sample, a light having a wavelength with a sufficiently small half width half maximum is employed.
In the following, a specific example of a microscopic system employing a light in the deep ultraviolet region.
FIG. 4 is a block diagram for showing the constitution of a confocal laser scanning microscopic system.
A confocal laser scanning microscopic system (a confocal laser scanning microscope) 100 comprises a laser light source 101, a microscope main body 110, and an optical image system 130.
The laser light source 101 emits a laser light of the deep ultraviolet region (the wavelength of 200 nm to 300 nm).
The microscope main body 110 comprises a beam expander 112 for enlarging a laser light 101a to a light beam 112a which has a sufficient size to cover the pupil plane of an objective lens 111, a beam splitter 114 which does not transmit the laser light, but transmits therethrough a light reflected by a sample 113, a two-dimensional scanner unit 115 for two-dimensionally scanning the laser light , a relay lens 116, a collective lens 117, a pin hole plate 118 which is disposed at a position conjugate to the focal plane of the objective lens 111 and is formed with a pin hole 118a for transmitting therethrough only a light collected by the collective lens 117, and a photo detector 120 for detecting the light transmitted through the pin hole 118a so as to convert such light into an electric signal.
The optical image system 130 comprises an image processing unit 131, a monitor 132, and the like, for forming an image of the sample 113 on the basis of the electric signal from the photo detector 120.
Note that the microscope main body 110 is mounted on an anti-vibration table 140 for making up for a high image quality.
An operation of the confocal laser scanning microscopic system having the aforementioned structure will be described below.
The laser light 101a emitted from the laser light source 101 is guided onto an optical path by means of reflection mirrors 102 and 103, is transmitted through the beam expander 112, then is reflected by the beam splitter 114. After that, the laser light is two-dimensionally scanned by the two-dimensional scanner unit 115, and is irradiated as a spot 119 on the focal plane on the sample 113 by means of the relay lens 116 and the objective lens 111.
The light reflected by the spot 119 goes back on the optical path to the objective lens 111, the relay lens 116 and then to the two-dimensional scanner unit 115, so as to pass through the beam splitter 114.
The light passing through the beam splitter 114 is collected on the pin hole 118a by the collective lens 117, is converted into an electric signal by the photo detector 120, and is displayed on the optical image system 130 as an image.
Since only the light on the focal plane of the sample 113 passes through the pin hole 118a, an unnecessary diffused light is removed by the pin hole 118a, so that it is possible to obtain an image with remarkably improved resolution and contrast in the depth direction in the optical image system 130.
Incidentally, the laser light source 101 is not so small-sized as to be incorporated in the microscope main body 110 and a single mode optical fiber capable of propagating a deep ultraviolet light has not yet been put to practical use, so that the laser light source 101 is mounted on the anti-vibration table 140 together with the microscope main body 110.
FIG. 5 is a schematic view of a deep ultraviolet laser light source according to the prior art.
The deep ultraviolet laser light source 101 is provided with a laser radiation source (basic laser light generating means) 105 and a laser cavity (wavelength converting means) 106. The laser radiation source 105 radiates a laser light 101b having the wavelength of 532 nm, and the laser light 101b is guided to the laser cavity 106.
The laser cavity 106 incorporates therein BBO (B.sub.a B.sub.2 O.sub.4 : barium boric acid) crystal, and this laser cavity 106 converts the laser light 101b with the wavelength of 532 nm into the laser light 101a with the wavelength of 266 nm.
FIG. 6 is a block diagram for showing the constitution of a microscopic system of a wide-field type. In FIG. 6, the identical portions to those in FIG. 5 are given the same referential numerals and symbols and description thereof will be omitted.
A microscopic system (laser microscope) 200 of a Koehler illumination type (wide-field type) comprises a laser light source 101 for radiating a laser light, a microscope main body 210, and an optical image system 230.
The microscope main body 210 is provided with a beam expander 212 for expanding a laser light to a light beam 212a which has a sufficient size to cover the pupil plane of an objective lens 211, a condensing lens 221, a beam splitter 214 which does not transmit the laser light, but transmits therethrough a light 213a reflected by a sample 213, a second objective lens 217, and a photo detector 220 for detecting a light focused by the second objective lens 217 so as to convert such light into an electric signal.
The optical image system 230 comprises an image processing unit 231, a monitor 232, and the like, for forming an image of the sample 213 on the basis of the electric signal from the photo detector 220.
An operation of a microscopic system of the Koehler illumination type having the aforementioned structure will be described below.
The laser light 101a emitted from the laser light source 101 is, after passing through the optical fiber (or a fiber bundle) 104 and the beam expander 212, reflected by the beam splitter 214, and is irradiated uniformly on the sample 213 by means of the objective lens 211.
The light 213a reflected by the sample 213 goes back to the objective lens 211, the beam splitter 114, and then the second objective lens 217.
The reflected light 213a is collected by the second objective lens 217, is converted into an electric signal by the photo detector 220, and is displayed on the optical image system 130 as an image.
However, in the confocal laser scanning microscopic system 100, the laser light source 101 is not so small-sized as to be incorporated in the microscope main body 110, a single mode optical fiber capable of propagating a deep ultraviolet light has not yet been put to practical use, and the laser light source 101 is mounted on the anti-vibration table 140 together with the microscope main body 110, so that the anti-vibration table 140 is required to be large-sized and the reflection mirrors 102 and 103 for guiding the laser light to the microscope main body are required to be adjusted, which results in an increase of the manufacturing cost.
Also, in the microscopic system 200 of the Koehler illumination type, the laser light source 101 is not required to be mounted on the anti-vibration table 240, so that the anti-vibration table 240 is not required to be large-size, unlike in the confocal laser scanning microscopic system 100. However, an optical fiber of an expensive quartz, fluorite, or the like, is employed as the optical fiber (or the fiber bundle) 104, which results in a high manufacturing cost.